In offset lithography, an image is present on a printing plate as a pattern or "image " of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a typical sheet-fed offset press system, the imaged plate is loaded onto a plate cylinder, where it is inked. The plate is then brought into contact with the compliant surface of a blanket cylinder and the image is transferred (i.e., offset) to the blanket cylinder. The blanket cylinder, in turn, applies the image to the printing medium (e.g., paper sheets) which are brought into contact with the blanket cylinder by an impression cylinder.
Although the printing plates were traditionally imaged photographically, more recently, a number of electronic alternatives have been developed for placing the image onto the plate. These digitally controlled imaging devices include lasers that chemically alter or destroy one or more plate layers, ink jets that directly deposit ink-repellent or ink-accepting spots on a plate blank and spark or ion discharge devices which physically alter the topology of the plate blank. These various methods of imaging lithographic plates are described in detail in U.S. Pat. Nos. 3,506,779; 4,054,094; 4,347,785; 4,911,075 and 5,385,092, among others.
These methods, moreover, may be used with the printing plate already mounted to the plate cylinder. That is, a plate blank may be loaded onto the plate cylinder and then imaged, thereby reducing press set-up time considerably. The plate cylinder typically includes two plate clamps mounted to the cylinder that extend longitudinally along its surface. To load a plate blank onto the cylinder, the leading edge of the plate is secured to one clamp and the plate is wrapped around the surface of the cylinder. The trailing edge of the plate is then secured to the other clamp. The space immediately between the two clamps where no plate material is present is referred to as a "void" segment.
Referring to FIG. 1, a printing station 100 comprising a printing plate 112 loaded onto a plate cylinder 114 is typically controlled by a press computer 116. The computer 116 is interfaced with an image control unit 118, which, in turn, is interfaced, via drivers 120, to a moveable writing head 122. The control unit 118 regulates the timing with which imaging data is supplied to the drivers 120 and, ultimately, the writing head 122. The writing head 122 is mounted for movement on a lead screw 124 and a guide bar 126. That is, the writing head 122 travels axially along the lead screw 124 such that the head 122 remains adjacent to the printing plate 112.
To image the plate 112, the plate cylinder 114 is rotated at a constant angular velocity as shown by arrow A. As mentioned above, the cylinder 114 typically includes a void segment 128 having means, such as plate clamps (not shown), for securing the ends of the printing plate 112. An angular encoder 130 is coupled to one end of the cylinder 114 and the image control unit 118. While the cylinder 114 rotates, the writing head 122 receives signals from the drivers 120 and, in response, one or more imaging devices (not shown) disposed inside the head 122 will either activate or not activate, depending on the data received from the drivers 120. The writing head 122 will thus image a circumferential path or swath about the plate 112, preferably from a leading edge 112A of the plate 112 to a trailing edge 112B.
When the writing head 122 passes over the void segment 128, the imaging devices are idled and no imaging occurs. It is at this point that the writing head 122 is preferably advanced in the axial direction in preparation for further imaging of the plate 112. That is, a stepper motor 132 under the control of the press computer 116 preferably turns the lead screw 124 thereby shifting the axial position of the writing head 122 along the guide bar 126. The writing head 122 is thus indexed during the "down time" that the head 122 is over the void segment 128. Thus, by the time the leading edge 112A of the plate 112 once again passes under writing head 122, the writing head 122 has been moved to the appropriate axial location to begin the next imaging pass. This process is repeated, causing the image to "grow" axially along the plate, until the writing head 122 has scanned the entire surface of the printing plate 112. The imaged plate 112 is then ready for printing.
The presence of a void segment, although useful for advancing the writing head, can prove disadvantageous for certain printing applications. First, the void segment effectively reduces the amount of plate cylinder surface area that is otherwise available for printing. Furthermore, the void segment prevents printing of continuous, unbroken images along a web or strip of print material. This is often desirable in order to produce decorative items such as wallpaper. In addition, the presence of a void segment presupposes the use of clamping mechanisms for the ends of the plate as well as alignment and control assemblies to ensure proper registration of the plate on the cylinder, thereby increasing the complexity of the cylinder.
To overcome these limitations, efforts have been directed at producing what are known as "seamless" printing members. As set forth in U.S. Pat. No. 5,440,987, which is co-owned by the assignee of this application and incorporated herein by reference in its entirety, a seamless printing member, as its name implies, is a continuous sheet of plate material, typically in cylindrical form. The seamless member preferably includes a hollow supporting cylinder or sleeve that fits over a plate cylinder. Surrounding the sleeve is a base layer of material that is susceptible to imaging techniques. The base layer, for example, may be polymeric in nature and characterized by efficient, ablative absorption of infrared radiation ("IR"). Surrounding the IR base layer may be a surface coating whose affinity for ink is the opposite of that exhibited by the sleeve. By selectively removing the surface coating through ablation of the IR layer, a pattern of spots (i.e., an image) having different affinities for ink may be created.
The seamless printing member may be integral with the plate cylinder itself, or secured to the plate cylinder by any suitable means. For example, the plate cylinder may contain an array of air capillaries that extend radially through its outer surface. Air introduced from a compressed source into the interior of the cylinder is directed radially outward from the cylinder's surface, expanding the diameter of the seamless printing member to ease its loading onto the cylinder. When the printing member is fully installed, the air flow may be stopped, causing the seamless printing member to relax to a tight fit over the cylinder. The seamless printing member is then ready for imaging and printing. The seamless printing member may be imaged completely from one edge thereof to the other, or a slightly narrower imaging region may be defined that provides a left and right margin on the seamless printing member.
Despite its advantages in terms of actual printing, utilization of a seamless printing member nevertheless complicates the imaging process. Without a void segment there is no predefined area over which the writing head may be advantageously advanced. Thus, alternative imaging techniques need be developed. One approach is to place the image on the seamless member in a helical pattern. This may be accomplished by moving the writing head axially relative to the cylinder at a constant, uniform speed while the cylinder is similarly rotated at constant speed. This approach, however, is disadvantageous in that the resulting image may be skewed slightly due to the helical imaging process. Furthermore, since the image information is often provided in an x-y format (i.e., a right angle coordinate system), imaging the plate in a helical manner may impose added complexity to the imaging process.